With the development of communication technologies, the reliability of communications becomes more and more important. One important influencing factor of the reliability of communications is network survivability. The network survivability refers to the capability of a network to maintain an acceptable QoS level during a network failure or an equipment failure. The main technical indices for characterizing the network survivability includes: redundancy, restore rate and restore time. The redundancy is defined as the ratio of the total idle capacity to the total working capacity of a network, which is mainly used for measuring the extra cost the system needs to pay for improving the survivability. The restore rate refers to the ratio of the number of restored channels to the total number of the originally failed channels, or the ratio of the restored capacity to the total capacity failed originally. The restore time refers to the time the network needs to expend to recover a failed service.
In the field of communications, the requirements to the restore time of a failure in different services are totally different. Generally, the ATMs (Automatic Teller Machines) of a large financial institution and a bank have the strictest requirements to the service restore time, usually less than 50 ms; ordinary communication services have relatively higher bearing capability on the service interruption time, but usually no longer than 30 minutes. These services are transferred on an optical network after being processed by a switch or a router. Usually, when the interruption time of a transport network is between 50 and 200 ms, the probability of losing the connection of switching service is less than 5%, and the influence to the No. 7 signaling network and cell relay service basically can be ignored. When the interruption time of the transport network is increased to 200 ms˜2 s, the probability of losing the switching service increases gradually. When the interruption time of the transfer network exceeds 2 s, most of the circuit switch connections, private lines and dialing services will be lost. When the interruption time of the transfer network reaches 10 s, the connection of all the communication sessions will be lost. If the interruption time of the transfer network exceeds 5 minutes, severe switch block will be caused, and the upper layer service will not be restored in a longer time.
Optical communication technologies develop rapidly in the field of communications currently, especially the advancement of optical devices boosts the development of optical communication technologies greatly. The transmission capacity is doubled every 9 months on the average, which is twice of the rate defined by Moore's Law. WDM (Wavelength Division Multiplexing) technology is a preferred technology for implementing high-speed and large-capacity transmission. At present, with the development of WDM technology, the communication capacity carried on a single fiber may reach Tbps level. In such a case, the fiber line failure or equipment failure on the bottom-layer optical network may usually affect a lot of services. As a result, it has become a focus to the carriers and the equipment vendors that how to improve the network survivability of the optical communication network.
WDM technologies become more and more mature with time, and its networking mode has developed from the back-to-back chain connection to the ring network and mesh network. The metropolitan area WDM is usually networked in the ring network mode. The SDH (Synchronous Digital Hierarchy) Ring Network provides protection modes such as: UPSR (Unidirectional Path Switching Ring), BPSR (Bidirectional Path Switching Ring), ULSR (Unidirectional Line Switching Ring), BLSR (Bidirectional Line Switching Ring) and SNCP (Sub-Network Connection Protection), etc. The WDM system provides similar protection modes such as: OUPSR (Optical Unidirectional Path Switching Ring), OBPSR (Optical Bidirectional Path Switching Ring), OULSR (Optical Unidirectional Line Switching Ring), OBLSR (Optical Bidirectional Line Switching Ring), OSNCP (Optical Sub-Network Connection Protection) and OCh-SPRing (Optical Channel Shared Protection Ring).
The so-called “Optical Channel Shared Protection” means that the bidirectional service connections on different segments of a ring share the same pair of wavelengths λ1 and λ2, which exist on two different fibers respectively. Meanwhile, the corresponding λ2 and λ1 on the two fibers are used as the protection wavelength for the working wavelength λ2 and λ1. Because the bidirectional service connections on different spans may share the same pair of wavelengths as the protection wavelengths, such a protection mode is referred to as the optical channel shared protection.
For a node that participates in the optical channel shared protection, three operations should be supported: service Pass Through, service add and service drop. The service Pass Through means that the protection service of other sites may pass through on this site directly, so that the protection service may be delivered to its destination node correctly. The service add means that the local service may be correctly switched to a backup channel for transmission when the local service is affected. The service drop means that a service with the local destination may be delivered to the local destination via a backup channel when the working channel is affected by a failure, and the service on the backup channel may be locally led to a receiver correctly.
In order to realize the above three operations on the node, in a usual optical channel shared protection method, a pair of wavelengths are first separated via an OADM (Optical Add Drop Multiplexer), and then the pair of wavelengths are processed in an optical channel shared protection switching device. FIG. 1 is a schematic diagram showing the internal structure of an existing optical channel shared protection node. Wherein, letter W refers to the working wavelength, letter P refers to the protection wavelength, letter D refers to the dropped service, and letter A refers to the added service, wherein the working wavelength and protection wavelength are distinguished by solid lines and dashed lines.
A usual method for implementing the optical channel shared protection will now be illustrated in conjunction with FIG. 1. In a typical existing optical channel shared protection node shown in FIG. 1, each wavelength may be configured as follows: the west-oriented outer fiber wavelength W11 employs wavelength 1 as the working wavelength, while the west-oriented inner fiber wavelength W21 employs wavelength 2 as the working wavelength. The east-oriented outer fiber wavelength W12 employs wavelength 1 as the working wavelength, while the east-oriented inner fiber wavelength W22 employs wavelength 2 as the working wavelength. Meanwhile, the west-oriented outer fiber wavelength P22 employs wavelength 2 as the protection wavelength of W22, the west-oriented inner fiber wavelength P12 employs wavelength 1 as the protection wavelength of W12, the east-oriented outer fiber wavelength P21 employs wavelength 2 as the protection wavelength of W21, and the east-oriented inner fiber wavelength P11 employs wavelength 1 as the protection wavelength of W11. Thus, a pair of wavelengths is shared by the bidirectional services on two segments of the fiber connected via the node, so that the optical channel shared protection may be realized. It can be seen that in this technical solution, a plurality of services on different segments may employ the same wavelength. In other words, a backup wavelength may be shared by a plurality of services on different segments, and the protection may be realized. Therefore, it is called the optical channel shared protection. In the node, after the wavelength pair is demultiplexed from the east and west line via an OADM, the wavelength pair can only be multiplexed again via an OADM and sent out in east and west line after being processed by an optical channel shared protection switching unit. The service add and service drop are also accomplished via the optical channel shared protection switching unit, wherein, the node shown in FIG. 1 accomplishes the add of the service A2 and the drop of the service D1 on the outer fiber, and accomplishes the add of the service A1 and the drop of the service D2 on the inner fiber. When a failure occurs on a certain segment of the fiber or on a node of the ring, the receiving end automatically switches to select and receive the signal from the other direction after the receiving end detects that the signal from one direction is lost. For example, when a failure occurs on the west-oriented outer fiber of the node shown in FIG. 1, the OADM automatically switches to receive the signal (P11) from the east-oriented inner fiber when the OADM detects that the outer fiber signal (W11) to be received from west is lost, wherein P11 acts as the protection wavelength of W11. Thus, the service protection may be realized.
In the above optical channel shared protection, a pair of wavelengths are taken as a basic unit. First of all, the wavelength pair is demultiplexed from the east and west lines via the OADM, then the wavelength pair is processed by an optical channel shared protection switching unit, and finally the wavelength pair is multiplexed again via the OADM and sent out in the east and west line. The disadvantage of the solution lies in that: when on a ring the optical channel shared protection needs to be implemented for a plurality of wavelength pairs at the same time, the corresponding OADM site and optical channel shared protection unit need to be arranged for each wavelength pair. Thus, the cost and complexity of the optical channel shared protection will be increased greatly.
To overcome the inconveniences in the process for realizing the optical channel shared protection and to achieve a protection of higher efficiency, a plurality of wavelengths may be combined into a group, and the shared protection will be implemented on the wavelength group. Because the same process is performed for a group of wavelengths, the protection switching will be relatively simpler. However, it is required that the protection switching actions adopted for different wavelengths in the same wavelength group should be consistent with each other, the different wavelengths should have the same source and destination nodes. However, in practice, the possibility to deploy the same service connection on a plurality of spans on the ring for a plurality of wavelengths is small, so the practicality of this method is limited.
In addition to the group shared protection, there also exists a subband multiplex section shared protection, which protects all the wavelengths. This protection mode borrows ideas from the feature of the optical channel shared protection that different wavelengths are used for the bidirectional service connections respectively. A wavelength band such as red band is used on the outer fiber, and another wavelength band such as blue band is used on the inner fiber, wherein the blue band wavelength on the outer fiber is used as the protection wavelength of the blue band wavelength on the inner fiber, and the red band wavelength on the inner fiber is used as the protection wavelength of the red band wavelength on the outer fiber. Apparently, this mode has the same wavelength utilization efficiency as that of the optical channel shared protection wavelength, and on the whole ring, it is only required that the same protection switching units are configured for all the nodes, thus the protection switching for all the wavelength pairs may be realized. When a failure occurs, it only needs to perform protection switching on the node adjacent to the failed segment, thus it is simple and convenient. However, because the subband shared protection still belongs to the multiplex section shared protection, during protection switching, the protected wavelength needs to travel around the protection fiber, which means that the distance the wavelength passes during the protection switching will be much longer than that during a normal working process. Although during the protection switching the longest distance may be less than 1.5 times of the length of the ring via careful wavelength planning, in a WDM system, it means that sufficient OSNR (Optical Signal to Noise Ratio) budget should be reserved for each wavelength connection. Especially in a WDM system with a long span, for this protection method, the OSNR budget and wavelength need to be planned carefully. For this method, there exist a lot of constraint conditions, and the implementing process is complex.